A number of attempts have been made to design and fabricate high precision optical sensing systems in which a position sensor is coupled to an optical source and detector using optical fibers. In one known type of system, an encoder is attached to a movable member, and the position of the member is determined by optically interrogating the encoder. The member may be one that rotates, in which case the encoder may comprise a disk that rotates with the member, or the member may be linearly movable, in which case the encoder moves linearly along with the member. The position of the movable member may itself be the desired measurand, or the system may be designed such that the member moves as a measurand (e.g., temperature or pressure) varies.
In a digital encoding system, the encoder includes a number of parallel coded tracks, each of which represents a specific bit in a binary word. Each track comprises a series of elements, each of which has an optical property that can assume one of two states, such as transmitting or nontransmitting. For each position of the encoder, the tracks will present a different set of elements, and therefore a different binary word, to the optical interrogation system. The precision of the system is limited only by the highest achievable element density of the least significant track.
Wavelength division multiplexing (WDM) may be used to permit an optical interrogation signal to be coupled to the sensor along a single fiber-optic cable, and to permit the resulting encoded signal to be returned to a detector along a single fiber-optic cable. The optical interrogation signal is formed such that it comprises light in a plurality of different wavelength bands. At the sensor, the interrogation signal is demultiplexed, such that each wavelength band strikes a different track. In an analog encoder system, the encoder includes a track that has a continuously variable optical property, or an optical property that varies in a relatively large number of steps. An interrogation signal is transmitted to the encoder along an optical fiber, passes through the variable density track, and then is returned to a suitable detector. Since the attenuation of the optical signal passing to and from the sensor is generally unknown, the interrogation signal generally comprises two component signals having different wavelength bands. At the sensor, the interrogation signal is demultiplexed, with one encoder system passing through the track, and the other component system bypassing the track. At the detector, the component signals are again demultiplexed, to provide an attenuation measurement that is relatively insensitive to fiber link losses.
Efforts have been made to develop a WDM system using laser diodes or light-emitting diodes to generate the interrogation signal. Lasers have very narrow bandwidths, typically less than five nanometers. By coupling the light from several lasers selected to emit at wavelengths that match the desired bands, the tracks of the sensor can be sequentially interrogated. However, this system fails if temperature excursions cause one or more laser emission wavelengths to drift out of their defined band. Other disadvantages of a laser system are the high costs and control circuit complexity associated with multiple lasers.
An alternative to a matched-band laser source is to use a broadband source, emitting for example over a range of 150 nanometers or more. Incandescent lamps, or several LEDs emitting at separate wavelengths, can be used to provide such a source. Typical LED spectra are 40 nanometers wide, and four LEDs would therefore be necessary for a 150 nanometer spectrum. The broadband light is modulated by the sensor, and the modulated signal is then dispersed onto a photodiode array for conversion to electrical signals. The disadvantage of this system is the cost and complexity associated with the photodiode array.